This invention relates to a circuit arrangement with a load connected to the output of an amplifier and a voltage generator having a d.c. output that delivers the supply voltage for at least the output stage of the amplifier.
Such circuit arrangements are used, for example, in an NMR tomography unit for the generation of magnetic gradient fields by means of a gradient coil arrangement which in this case forms the load on the amplifier. FIG. 1a shows the typical time curve of the magnetic flux for such a gradient coil arrangement. If the eddy current effects are disregarded, then the current through this coil arrangement has the same curve. Its amplitude in this case is typically 200 A.
FIG. 1b shows the time curve of the voltage at the gradient coil arrangement which is necessary for the current curve shown in FIG. 1a. During the rise and fall of the current two voltage pulses of relatively high amplitude (of the order of 70 V) have to be produced. Between these pulses there has to be a voltage which is just sufficient to compensate the active resistance loss of the coil arrangement.
The supply voltage for the amplifier feeding the coil arrangement must be slightly higher than the peak voltage at the gradient coil arrangement. The power loss in the amplifier is given by the integral of the product of the current as shown in FIG. 1a and the difference between the supply voltage and the voltage as in FIG. 1b. It is particularly large if the current through the coil arrangement is large and the voltage at the coil arrangement is small.